1. Technical Field
The present invention pertains to navigation systems. In particular, the present invention pertains to compensating for errors of inertial instruments and providing stochastic control within a navigation system (e.g., GPS, etc.) in order to obtain accurate navigational information in a signal challenging environment and provide rapid re-acquisition of a satellite signal after signal loss.
2. Discussion of Related Art
A Global Positioning System (GPS) receiver acquires and/or tracks a satellite signal by generating a replica signal of a signal received from the satellite and correlating the replica signal with the signal currently being received from the satellite. The replica signal is advanced or delayed in time until correlation with the received signal is attained. Generally, this process is performed by respective tracking loops for a code (e.g., pseudorandom noise code (PRN)) and the corresponding carrier signal. A code tracking loop drives the code phase of a replica signal to be in alignment with the code phase of the received signal, thereby enabling coherent demodulation of the received signal.
The tracking loops adjust the phase and the frequency of the generated replica signal for numerous changing variables (e.g., user and satellite relative motion, user clock drifts, etc.), and typically include a signal generator or numerically controlled oscillator, a correlator, a discriminator or error generator and a controller. The signal generator generates an estimated replica signal of the received signal based on a control signal that advances or delays in time the replica signal relative to the received signal. The correlator multiplies the received signal by the replica signal and passes the multiplied signal result through a low pass filter. The discriminator generator generates an error or discriminator signal having a value related to the difference between the received signal and the replica signal, while the controller filters the discriminating signal into the control signal that is provided to the signal generator.
The tracking loops basically determine the error signal that is a measure of the range and range rate difference between the generated and received signals. The error signal is processed by a transfer function that generates input values for the signal generator to advance or delay the generated replica signal. Once the replica signal correlates with the received signal, the tracking loop is in lock and the error signal provided to the signal generator is near zero. In this case, the state of the generation process for the code and carrier signal can be sampled to obtain a measure of the pseudorange and pseudorange rate. The pseudorange is the algebraic sum of the geometric range between the transmit antenna and receive antenna, and a bias due to a user and satellite vehicle clock error. The pseudorange rate is approximated by the sum of the amount of range change for a predetermined amount of time and a bias due to the drift of the user oscillator frequency. The psuedorange and/or pseudorange rate measurements are utilized to estimate navigational information (e.g., position, velocity, etc.).
A navigation receiver should be able to withstand signal interference with respect to signal acquisition and tracking. Accordingly, inertial navigation systems have been combined with GPS signal tracking and acquisition systems in tightly coupled and ultratightly coupled architectures for enhanced tracking and acquisition of the received signal. Initially, inertial navigation systems (INS) include an inertial measurement unit (IMU) for processing inertial measurements. Inertial aiding is utilized to apply the data from the IMU to assist the tracking loops (or the extrapolation of the user position and velocity).
With respect to a tightly coupled architecture, the relative motion between the satellite antenna and the user antenna may be predicted to a certain accuracy based on the IMU measurements and the position and velocity of a satellite evaluated from the satellite ephemeris data (e.g., status and location information). The GPS with inertial aiding uses known user relative motion from IMU measurements and the satellite position and velocity data evaluated from the ephemeris data to compute the line-of-sight rate between the user antenna and the GPS satellite for providing rate data to aid the tracking loops. The pseudorange and psuedorange rate measurements from the GPS receiver are utilized (instead of the processed position and velocity measurements from the INS) to produce navigational information (e.g., position, velocity, etc.).
An ultratight coupling (UTC) technique is disclosed in U.S. Pat. No. 6,516,021 (Abbott et al.). This type of technique drives signal generators for each satellite channel using information from the current best estimate of a navigation state vector and satellite position and velocity predicted from the satellite ephemeris data. The navigation state vector contains best estimates of user position, velocity, attitude, receiver clock errors and calibration coefficients for the IMU. The best estimate of the navigation state vector is based on a previous major cycle of an integration Kalman filter, and IMU samples from a previous major cycle update. In the ultratight coupling technique, the quadrature I and Q samples from a correlator are sent to a Kalman pre-filter that processes the samples and produces a measurement residual error vector needed by the integration Kalman filter based on the errors in the code and carrier replica signals provided to correlators (rather than an absolute measurement of pseudorange or pseudorange rate). The integration Kalman filter uses the best estimate of the predicted navigation state vector to generate samples of the predicted pseudorange and the pseudorange rate for the major cycle of the integration Kalman filter. This predicted pseudorange and pseudorange rate information, in conjunction with the satellite ephemeris information and inertial measurement data, are used to generate and update the code and carrier replica signals used in the correlation process for improved tracking and accuracy. Thus, the IMU measurements, time propagation using the user oscillator and satellite ephemeris data are used to drive the correlation process for replica signal generation. Since these items are not vulnerable to interference or jamming, the ultratight coupling technique provides interference tolerance.